1. Field of the Invention
This invention is in the field of non-destructive testing by means of the penetrant inspection process and is specifically directed to an improved dry developer composition for use in such process.
2. Description of the Prior Art
The penetrant method for detecting surface discontinuities or flaws has been extensively used for the inspection of wrought and cast products of both ferrous and non-ferrous metals, powder metallurgy parts, ceramics, plastics, and glass objects. In the typical penetrant inspection process, a liquid penetrant which may be oil based or water based is applied to the surface of the workpiece and allowed to penetrate into any flaws which extend to the surface. The penetrant composition may contain a flourescent pigment or a visible dye. After a sufficient dwell time, excess penetrant is removed from the surface by a remover, usually an emulsifier, which is compatible with the vehicle of the penetrant. The next step consists in applying a developer to the surface to assist in drawing out portions of the penetrant which have become lodged in surface flaws to render the same contrastingly visible to the background. An inspection is then made using either ultraviolet light in the case of a flourescent penetrant or ordinary white light in the case of a visible dye.
Both dry developers and developers consisting of suspensions in a fluid have been used in the penetrant inspection process. For any penetrant to be effective, it should have a combination of various physical properties. First, the developer must be absorptive toward the penetrant so as to maximize the blotting effect. It must have a fine grain size and a particle shape that will disperse and expose the penetrant at a flaw over as large a surface area as practical, while producing strong and sharply defined indications of flaws. The developer must be capable of effectively masking out interfering background colors and of providing a contrast background for indications, especially when color contrast penetrants are used. The developer must be easy to apply and it must form a thin uniform coating over a surface. The developer composition should be easily wetted by the penetrant at the flaw. In the case of fluorescent penetrants, the developer itself should be non-flourescent. It must be easy to remove after inspection and it must not contain ingredients harmful to parts being inspected or to equipment used in the inspection operation. Furthermore, it must not contain ingredients which are harmful or toxic to the operator.
Dry developers were the first developers to be used with fluorescent penetrants and are still widely used therewith. The first powders used were a simple mixture of chalk and talc which gave reasonably good results. As penetrant inspection became more widely used, however, these powders were no longer considered very practical. Instead, much lighter material such as amorphous silica powders were used which proved to be superior in many ways.
The ideal dry developer powders should be light and fluffy and should cling to dry metallic surfaces in a fine film. Adherence of the dry powder should not be excessive, however, because the amount of penetrant at fine flaws is so small that it cannot work through a thick coating of powder. The powder should not float and fill the air with dust.
Ideally, the dry developer powders should not be hygroscopic. If such powders pick up moisture when stored in areas of high humidity, they will lose their ability to flow and dust easily, and may pack or lump up in containers or in developer bins.